Natural gas liquefication involves the conversion of natural gas to liquid form to facilitate transportation and storage of the gas. Current commercial cryogenic processes for making liquefied natural gas (LNG) include the steps of compressing a refrigerant and flowing it through a spiral wound or brazed aluminum heat exchanger. In the heat exchanger the refrigerant exchanges heat with the natural gas and liquefies the natural gas. These heat exchangers are designed to provide very close temperature approaches between the refrigerant and natural gas streams that are exchanging heat. Increasing the thermal efficiency of these heat exchangers through changes in design or materials of construction typically results in increasing the capital cost of the heat exchanger, increasing the pressure drop for the refrigerant flowing through the heat exchanger, or both. Increasing the pressure drop results in increased compressor requirements. The compressor service required for these processes comprises a significant portion of the capital and operating cost of these processes. The problem therefore is to provide a process that results in a reduction in the pressure drop for the refrigerant flowing through the heat exchanger. This would improve the productivity and economics of the process. The present invention provides a solution to this problem.
Due to the large capital cost of cryogenic liquefaction, LNG plants are being built with ever-larger capacities in order to meet project economic targets through economies of scale. This need for economies of scale has resulted in increases in the size of single-train LNG processes. Currently, the size of a single-train LNG process with one compressor is limited by the maximum size of the compressors that are available. The problem therefore is to reduce the compressor requirements for these processes in order to increase the maximum size for the LNG process that is possible. This invention provides a solution to this problem.
Aluminum is typically used as a material of construction in conventional cryogenic heat exchangers. Aluminum minimizes heat transfer resistance between fluid streams due to the fact that it is a high thermal conductive material. However, since it is a high thermal conductive material aluminum tends to decrease the effectiveness of the heat exchangers due to axial conduction. This limits the ability to shorten the length of these heat exchangers and thereby reduce the overall pressure drop. An advantage of the present invention is that it is not necessary to use high thermal conductive materials such as aluminum in constructing the heat exchanger used with the inventive process.